This invention relates to a closed loop LPP ignition timing control for an internal combustion engine suitable for a motor vehicle. In such a control, the location, in crankshaft angle relative to top dead center, of peak combustion pressure in the combustion chamber (LPP), or a computed average peak combustion pressure (ALPP), is compared with a desired location of peak pressure (DLPP); and an engine ignition parameter such as spark timing is adjusted to return LPP toward the desired value and thus maintain MBT ignition timing. DLPP is generally about 15 degrees ATDC, although this may vary slightly among different engines or in engine operation.
An LPP ignition timing control of this type is shown in the U.S. Pat. No. 4,481,925, to Karau et al, issued Nov. 13, 1984. In this system, the signal from a combustion pressure sensor is processed by a peak detector and an LPP signal generated to control engine spark timing in a closed loop feedback control. However, the accurate operation of a closed loop system as shown in Karau et al U.S. Pat. No. 4,481,925 depends upon the presence of certain events and relationships which are generally assumed but are, unfortunately, not always present for all engine operating conditions. For example, a detectable peak combustion pressure must exist and be accurately assignable to a particular crankshaft angular position. The LPP point must not be varying too greatly or quickly for stable closed loop control. Changes in ignition timing must produce predictable changes in LPP and must be capable of bringing LPP to the desired value, DLPP.
Although these conditions are true for most operating modes of a typical internal combustion engine, they can not always be relied upon. In particular, conditions of high dilution from exhaust gas recirculation or lean mixture, greatly retarded spark and light engine load result in slower burning combustion with a low, flat combustion pressure curve. Since most internal combustion engines have a plurality of combustion chambers, fuel tends to be unevenly distributed among them, even with the most careful design. As combustion becomes more marginal due to a high average dilution level, the percentage of misfires in the leanest combustion chambers becomes higher. This leads to wide variations in the locations of detected pressure peaks or an increase in the percentage of nonexistent or undetected peaks.
In addition, the relationship between LPP and ignition timing becomes adversely affected. In normal combustion the two are related over most of the usable range of ignition timing in a relatively well behaved, single functional manner, which may even be approximately linear. An example of a curve expressing such a relationship is shown in FIG. 2. Thus a simple closed loop control is adequate to maintain LPP at the desired level DLPP. However, in engine operating modes involving high fuel dilution and light engine loads, the curve of the relationship can appear as in FIG. 4, with a fold-over effect in the retarded spark region which produces an ambiguity in the relationship and a reversal of the slope of the curve in a portion of the region which makes closed loop control difficult. In some modes the curve can even fall completely below the desired LPP level DLPP, as shown in FIG. 6, in which case, control to DLPP is impossible.
These problems have been discussed, to the knowledge of the inventor, in only one place in the prior art: the U.S. Pat. No. 4,596,218 to Karau et al, issued on June 24, 1986. In this patent these problems are recognized; and an LPP combustion control is described in which a sensed abnormal combustion condition causes the control to change ignition timing toward predetermined stored timings and substitute temporary DLPP values in the closed loop control algorithm for the duration of the sensed abnormal combustion condition. The temporary DLPP values are derived from measured LPP so as to drive the LPP as closely as possible toward the actual desired value, DLPP. At the end of the sensed abnormal combustion condition, the temporary DLPP values are continued until the temporary and actual DLPP values converge. Abnormal combustion may be sensed as excessive variation in LPP, excessively advanced average LPP or excessive numbers of undetected pressure peaks.
Unfortunately, although the apparatus of Karau et al U.S. Pat. No. 4,596,218 was a great improvement over the prior art in handling such abnormal combustion in an LPP ignition timing control, it was discovered that there was at least one light throttle engine operating mode which it did not always handle correctly. Although some minor modification of the apparatus may have taken care of the matter, such modification was found to be unnecessary upon the invention of the apparatus described and claimed herein.